A known typical voice coil actuator is comprised of a core, an axially polarized cylindrical magnet, a base plate, a rod, and two pole pieces. The rod and the magnet are coaxially mounted on the base plate, such that the rod extends through the center of the magnet. The two pole pieces are coaxially mounted respective to the magnet and the rod. The first pole piece has an inner diameter less than the inner diameter of the cylindrical magnet and the second pole piece has an inner diameter greater than the diameter of the rod. Therefore, a gap remains between the facing sides of the pole pieces. A coil is coaxially suspended within the gap.
Referring now to FIG. 1, there is shown a novel voice coil actuator 10 assembled according to the principles of the present invention and disclosed in co-pending U.S. application No. 07/740,068, which is incorporated by reference herein. The actuator 10 includes a core member 12, a pole piece 14, a radially polarized magnet 16 and an electrical current conductive coil 18. The core member 12 and the pole piece 14 are preferably constructed from magnetic flux conductive material. The core member 12 is cylindrical and has an upper surface 20 with a continuous chamber 22 disposed in the upper surface. The chamber 22 has an outer first wall 24 having a predetermined chamber diameter. The pole piece 14 is preferably a cylindrical rod. The cylindrical rod is coaxially mounted in the chamber 22 and has a first surface 26 that forms an inner second wall 28 of the chamber 22.
The magnet 16 is disposed in intimate contact with the outer first wall 24 so that a gap 30 remains between the magnet 16 and the inner second wall 28. Alternatively, the magnet 16 could be mounted to the inner second wall 28 and spaced from the outer first wall 24. The magnet 16 is annular or cylindrical in shape and has radial polarization. Accordingly, the magnet 16 has a first face of a first magnetic polarity adjacent the outer first wall 24 and a second face of a second, opposite magnetic polarity facing the gap 30. The magnet 16 has a lower edge 32 spaced from the bottom wall 34 of the chamber 22 and an upper edge 36 coextensive with the upper surface 38 of the pole piece rod so that the magnetic flux is substantially confined normal from the second face of the magnet 16 across the gap 30 to the inner second wall 28. Since the second face is a pole of the magnet, the flux emanating from this pole will be continuous and uniform across the entire surface of the second face and be of the same intensity at the lower edge 32 and upper edge 36. The flux in the gap of the novel voice coil actuator 10 is therefore uniform along the height of the gap. This uniformity of flux is a significant advantage of the novel actuator 10 exhibits over the prior art actuators.
The magnet 16 may be of unitary cylindrical construction. The magnet 16 may also be constructed from a plurality of arc segments 40 (shown in FIG. 2) of radially polarized magnetic material. Each arc segment 40 may be bonded directly to the outer wall 24 to form the annular construction. It is also possible to approximate an annular radially polarized magnet by a plurality of flat magnet segments placed side by side on the outer wall 24. The gaps remaining between each flat segment would be filled with a flux defocuser, such as iron.
The spacing of the lower edge 32 from the bottom wall 34 is selected to be larger than the width of the gap 30 between the magnet 16 and the inner second wall 28. For fringing from the lower edge 32 toward the bottom wall 34 to occur, the fringing field would have an axial component and a radial component. By having the radial component travel a much shorter distance than the axial component, the resistance to the radial component is considerably less than the resistance to the axial component. Since the flux will follow a path of least resistance, these spacings will force the flux to stay radial in the gap and not fringe from the lower edge. The magnet 16 therefore is bonded to the outer wall 24 at a predetermined height in order to eliminate fringing.
Although the actuator 10 may be constructed as described in U.S. patent application Ser. No. 07/740,068, an improved method and devices for use in assembling the actuator 10 have been discovered. The new method and devices overcome several of the disadvantages of the previous method of assembling the actuator. The first disadvantage relates to bonding the radially polarized magnet to the outer wall of the chamber. The magnet ring must be coaxially aligned with the chamber axis. Also, after the magnet ring is positioned within the chamber, the entire circumference of the magnet ring outer surface must either be in contact with or in close relation to the outer surface of the chamber. An adhesive is then applied between the magnet ring and the chamber outer wall. If the magnet ring is not coaxially aligned within the chamber, a portion of the magnet ring may be in direct contact with the outer wall of the chamber, or spaced from the outer wall, and the adhesive cannot be properly applied.
Another disadvantage arises when a plurality of magnet segments are bonded to the outer wall to form an annular shaped magnet. This disadvantage is caused by the fact that the magnet segments repel each other. Therefore, after N.sub.-1 segments are inserted into the chamber, the Nth magnet resists fitting into the chamber because the remaining N.sub.-1 magnets are forcing themselves out into evenly spaced distances around the ring. Therefore, a new method and device for bonding the ring magnet or magnet segments to the outer wall surface of the chamber is desired.
Another disadvantage with assembling the above-described actuator is related to assembling the pole piece into the chamber. The magnet ring is bonded to the chamber wall before the pole piece is inserted into the chamber. As a result, when the pole piece is inserted into the chamber it is attracted off-axis by the magnet, and is not easily coaxially aligned within the chamber. Therefore, a need exists for an improved method and device for easily and accurately assembling the pole piece into the chamber of the core.